His style has an element of what the Guardian calls discreet laddishness, and he often introduces a Three-Tenorsish knockabout touch to his performances. At the Wigmore Hall he got the audience to sing along to Molly Malone, and while giving Don Giovanni's serenade as an encore he wandered down the aisles distributing flowers to women in the audience.
There must be something about Welsh bass-baritones, for even cuddly Geraint Evans enjoyed exercising his talent for seduction on the stage, though of course if you're playing the Don that is what you are supposed to be doing. I once saw him on TV giving a master class in which he showed the student learning Zerlina's role not only how to sing it but also just what it felt like to have an elderly but irresistible lecher after you.
It's not only singers who can do this. The great Paul Tortelier gave a series of master classes on TV in the seventies; in the one I saw there was no doubt at all about the powerful effect he was having on the pretty young cellist, and the fact that he knew it perfectly well. He was in his sixties then with none of Terfel's Cardiff Arms Park kind of bravura and it is impossible to imagine him with a bare chest, but he achieved much the same result with his glittering eyes.